Composite Structure with Exposed Conductive Fiber for Static Dissipation, and Method for Making Same

ABSTRACT

Provided is a storage tank or structure with conductive fiber material (e.g. carbon fiber) for dissipating electrostatic charge. The tank comprises a tank wall made of composite material, such as polyester-fiberglass composite. On an inner tank surface, open areas are provided in which the conductive fiber is exposed. The conductive fiber material has broken fiber tips and stray fibers for collecting electrostatic charge. Outside the open areas, the conductive fiber material is covered with a layer of cured resin. The conductive fiber is exposed only in the open areas. An impermeable film may be present under the conductive fiber in the open areas. The present invention also includes a method for making the tank, in which a liquid, gel, or impermeable film mask is applied to the conductive fiber material. The mask functions to prevent infiltration of liquid resin into the conductive fiber material.

RELATED APPLICATIONS

The present application claims the benefit of priority of copendingprovisional patent application 61/851,028 filed on Feb. 28, 2013 andcopending provisional patent application 61/852,780 filed on Mar. 21,2013, which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to explosion prevention andelectrostatic charge dissipation. More particularly, the presentinvention relates to a composite storage tank having an integralconductive fiber material for electrostatic charge removal, and a methodof making the same.

BACKGROUND OF THE INVENTION

Liquid storage tanks are commonly used in petroleum production and atindustrial facilities. These tanks are used to store petroleum products,contaminated wastewater, or process chemicals. These materials maycontain flammable, volatile components that present an explosion hazard.If a tank contains flammable vapors and air, an electrostatic dischargecan trigger a dangerous and costly explosion.

Consequently, electrostatic drain devices are sometimes employed insidestorage tanks. The electrostatic drain device safely dischargeselectrostatic charges in the contained air and liquid to groundpotential, thereby eliminating the possibility of an electrostaticexplosion trigger.

FIG. 1 shows a conventional storage tank with electrostatic drainaccording to the prior art. The tank 10 contains a liquid and a mixtureof air and explosive vapors. The explosive vapors may comprise lowmolecular weight hydrocarbon vapors such as butane for example. Theliquid flows into and out of the tank 10 via a pipe connection 11. Asthe liquid moves through the pipe, electrostatic charge is created inthe liquid via well-known triboelectric effects. This electrostaticcharge will become trapped in the tank if there is no conductive path toground potential. The trapped electrostatic charge can trigger anexplosion of the air and flammable vapor mixture.

Nonconductive tanks (e.g. made of polymers or fiberglass) areparticularly problematic because they do not provide an electricallyconductive path to ground potential. Metal tanks can also present ahazard if they are coated with an electrically insulating coating ofepoxy or paint.

The prior art solution to this problem is to use a metal twisted wirebrush 12 as an electrostatic drain. The metal wire brush device 12 issuspended inside the tank 10 and electrically connected to groundpotential 13. The wire brush comprises a twisted cable 14 with embeddedsmall diameter wires 15 (e.g. 0.001-0.020″ diameter). The small diameterwires have sharp tips that serve to concentrate an electric field, andthereby facilitate charge collection. The wire brush 12 is typicallymade entirely of stainless steel. In operation, the drain deviceaccumulates electrostatic charge present in the liquid and air, andprovides a path for this charge to flow to ground potential 13.

The conventional solution of FIG. 1 is effective for dissipatingelectrostatic charge. However it has several serious disadvantages,including cost, susceptibility to corrosion, difficulty of installation(since the central twisted wire is rigid or semi-rigid), and tendency ofthe small wires to loosen and fall off over time. The small wires canloosen because they are held at only a single point where they passthrough the twisted cable. Hence if corrosion causes one wire todislodge, then all other wires in the same bundle will fall out as well.Small wires or corroded metal particles that fall into the liquid willdamage downstream equipment Consequently, the wire brush 12 presents asignificant hazard for liquid-handling equipment such as filters, valvesand pumps.

Corrosion is a great concern at petroleum facilities because the liquidsin the tank often contain combinations of salts, acids, hydrogensulphide and other substances that corrode many types of metals,including stainless steel. This is one reason why non-metallic tanks arepreferred for these applications.

Fiberglass tanks are corrosion resistant, but because they areelectrically insulating, fiberglass tanks are an explosion hazard.

FIG. 2 shows a conventional method for fabricating acomposite-fiberglass tank. A cylindrical mandrel 16 is used as a formthat defines the internal shape and dimensions of the tank. A fiberglassstrip 17 is soaked in curable resin (e.g. polyester resin) and wrappedaround the mandrel. The fiberglass strip can also be wrapped in adiagonal or zigzag fashion. After the mandrel is covered withresin-soaked fiberglass, the resin is cured, and the resulting tube isremoved from the mandrel. Bottom and top covers (not shown) are attachedto enclose the ends of the tube. A problem with this method is that allinside surfaces of the tank are electrically non-conductive due to athin layer of non-conductive resin coating the inside surfaces. Evenwhen conductive carbon fiber is used instead of fiberglass, the insidesurface of the tank is electrically non-conductive due to a layer ofresin.

It would be a great advantage and improvement in the art to provide acomposite fiberglass tank with integral electrostatic charge dissipationfunctionality.

SUMMARY

Provided is a charge-dissipating tank comprising a tank wall, aconductive fiber material embedded in the tank wall, and an open area onan interior surface of the tank wall. The open area overlaps theconductive fiber material, and the conductive fiber material is exposedin the open area. The conductive fiber material is covered with resin inareas outside the open area locations. The resin can be a resin used tomake the tank.

The conductive fiber material preferably has 1, 10, 20 or 100 brokenfiber tips of stray fibers per square inch in the open areas.

The conductive fiber material can be in a spiral around the tank.

An impermeable film can be disposed between the conductive fibermaterial and the tank wall. The impermeable film can be an artifact ofthe present method for manufacturing the tank.

The present invention also includes a method for making astatic-dissipating tank. The method includes the steps of: 1) applying amask to portions of a conductive fiber material, 2) disposing the maskedconductive fiber material on a tank mandrel, 3) disposing structuralfiber material (e.g. fiberglass, carbon fiber) and liquid resin (e.g.polyester resin, epoxy) on the masked conductive fiber material andmandrel, 4) curing the resin, 5) releasing the conductive fiberstructure from the mandrel, and 6) removing the mask material from theconductive fiber, thereby exposing the conductive fiber in previouslymasked areas.

Impermeable film can be used as a mask on one or both sides of theconductive fiber material. The mask can also comprise a liquid or gelmask material, such as polyvinyl alcohol, plant gums dissolved in water.Liquid mask preferably infiltrates into the conductive fiber material.

Alternatively, the mask material can be applied to the conductive fibermaterial while it is on the mandrel. For example, the impermeable filmcan comprise adhesive tape, which can also function to hold theconductive fiber material on the mandrel.

Removing the mask can also include removing some cured resin material.After assembly, a layer of resin material might cover the masked areasof the conductive fiber material.

The conductive fiber material can be applied to the mandrel by wrappingaround the mandrel (e.g. in a spiral), or laying strips (e.g. straightstrips) on the mandrel.

Alternatively, the mask material can comprise a water-soluble substance,and removing the mask can comprise rinsing the masked areas with water.

The mask material can comprise impermeable film such as adhesive tape.The impermeable film can be wrapped around the conductive fibermaterial, and the impermeable film can be used to attach the conductivefiber material to the mandrel.

The present invention also includes a method for attaching a conductivefiber material to a tank surface, for example for static dissipation.The method comprises the steps of: 1) applying mask to portions of aconductive fiber material, 2) disposing the masked conductive fibermaterial on the tank surface, 3) applying liquid resin to the conductivefiber material, 4) curing the resin, and 5) removing the cured resin andmask from masked areas of the conductive fiber material.

The mask can comprise a liquid or gel mask material, or impermeablefilm. The impermeable film can comprise adhesive tape for example. Theimpermeable film can also function to hold the conductive fiber materialon the tank surface, while liquid resin is applied.

The conductive fiber material can be attached to the tank wall before orthe same time as mask is applied.

The present invention also includes a charge-dissipating panelcomprising structural fiber embedded in a resin material, a conductivefiber material embedded in the panel, and an open area disposed on asurface of the panel, wherein the open area overlaps the conductivefiber material, and the conductive fiber material is exposed in the openarea.

DESCRIPTION OF THE FIGURES

FIG. 1 (Prior Art) shows a storage tank with a conventional stainlesssteel brush electrostatic charge dissipater.

FIG. 2 (Prior Art) shows a conventional method for making a fiberglassor carbon fiber tank.

FIG. 3 shows a static-dissipative tank according to the presentinvention.

FIG. 4 shows a close-up view of an open area with exposed conductivefiber according to the present invention.

FIG. 5 shows a piece of conductive fiber material with masked areas ofdifferent shapes.

FIGS. 6A-6F illustrates a method for making a tank according to thepresent invention.

FIG. 7 shows masked conductive fiber material wrapped around a mandrel,according to an alternative method for making a tank according to thepresent invention.

FIGS. 8A-8E illustrates a method for making a tank according to thepresent invention in which an impermeable film is used as a mask on theconductive fiber material.

FIGS. 9A-9E illustrate a method for making a tank according to thepresent invention in which impermeable film is applied to both sides ofthe conductive fiber material.

FIG. 9F shows a cross sectional view of conductive fiber material withimpermeable film surrounding the conductive fiber material. Theimpermeable film forms a tube around the conductive fiber material.

FIGS. 10A-10D illustrate a method for adhering a conductive fibermaterial to the inside surface of a tank, in which a mask is used toinhibit the spreading or capillary flow of resin in the conductive fibermaterial.

FIG. 11 shows a cross sectional view of a tank with a conductive fibermaterial adhered to an inside surface of the tank according to themethod illustrated in FIGS. 10A-10D.

FIG. 12 shows a static dissipative panel according to the presentinvention.

FIG. 13 shows cross sectional view of a step in a method for making thepresent invention in which the conductive fiber material is adhered tothe mandrel or tank surface with impermeable film comprising adhesivetape.

DETAILED DESCRIPTION

The present invention provides a storage tank with electricallyconductive fibers for collecting and removing electrostatic chargeinside the tank. The electrically-conductive fibers are exposed (i.e.not covered with non-conductive material such as resin) on insidesurfaces of the tank. When connected to a ground potential, theconductive fibers remove electrostatic charges from inside the tank. Thepresent tank is made by covering portions of the electrically conductivefiber with a mask material that inhibits infiltration by nonconductiveresin. The conductive fibers are wrapped around a tank mandrel, followedby wrapping structural fibers (e.g. fiberglass or carbon fiber) andresin, as known in the art. After removal of the tank from the mandrel,cured resin and mask is removed from masked areas of the conductivefiber, thereby exposing the conductive fiber on inside surfaces of thetank. The exposed areas of the conductive fiber function to collectelectrostatic charge. Cured resin does not adhere to the conductivefiber in masked areas. The mask can comprise high viscosity liquids,thixotropic gels, mold-release agents and/or impermeable films such aspolyester, polyimide, polyethylene, or polyethylene terephthalate film,or metal foil.

DEFINITIONS

Exposed: Lacking a nonconductive coating such as a resin coating (e.g.polyester or epoxy), or paint. The surface of exposed electricallyconductive fiber is electrically conductive.

Conductive fiber material: Fibrous having electrical conductivity andcontinuity sufficient for collecting electrostatic charge. Theconductive fiber material has electrical continuity, so that it cantransport electrical charge. Fiber diameter can be in the range of1-1000 microns for example. Suitable conductive polymeric fiber can bemade of carbon fiber, inherently conductive polymers, or polymercomposites comprising non-conductive polymeric matrix with additivessuch as carbon nanotubes, carbon black, metal particles, chopped carbonfiber or the like. Also, metallic fiber can be used.

Yarn: A collection of a plurality or large number of approximatelyparallel (over a long length scale), loosely twisted, woven oraggregated fibers. A yarn can comprise tangled fibers. A yarn willtypically comprise at least about 100 or 500 individual fibers. Forexample, carbon fiber yarns often contain 3000 or 6000 fibers.

Mask: A solid, liquid, gel, film or sheet material effective forpreventing infiltration of resin into the conductive fiber material.

Resin: A hardenable liquid material used to form the matrix of a fibercomposite material. Typical resin materials include polyester resin andepoxy.

Infiltrate: To flow into pores or interstices of a material, such as thespaces between individual fibers comprising the conductive fibermaterial. Infiltration may or may not be associated with wetting orcapillary action.

FIG. 3 shows a tank 20 according to the present invention, and amagnified view of an open area 22 on an inside surface of the tank. Thetank 20 has walls 21 with a thickness 24. The thickness can be about1/16″-1″ for example. The tank wall 21 can be made ofpolyester-fiberglass, carbon fiber-epoxy composite for example. The tankwalls 21 have embedded strips of conductive fiber material 26. In FIG.3, the conductive fiber material 26 is seen edge-on. The conductivefiber material 26 can comprise carbon fiber, or other electricallyconductive material. The conductive fiber 26 is exposed in the openareas 22. In the exposed open areas 22, the conductive fiber materialhas broken fiber tips 28 or stray fibers 29 projecting away from theconductive fiber material 26. The conductive fiber material 26 is notcovered with cured resin in the exposed open areas 22.

In locations outside the open areas 22, a resin covering 30 is present.The resin covering 30 is missing in the open areas. The resin covering30 can be made of the same resin comprising the tank wall. The resincovering can be vanishingly thin, or can have thicknesses of about0.005″, 0.010″ or 0.020″ for example. The thickness of the resincovering 30 will vary from place to place and in some places can beessentially zero. It is possible for the conductive fiber material 26 tobe present at the surface of the resin covering 30. However, the brokenfiber tips 28 and stray fibers 29 will generally not be present inlocations covered by the resin covering 30. Consequently, the resincovering 30 blocks electrostatic charge accumulation in areas where itcovers the broken fiber tips 28 and stray fibers 29.

As explained below, the resin covering 30 is mechanically removed (aftercuring) from open areas 22. Consequently, edges 32 of the open areas 22may show signs of ripping, cutting or shearing of the resin covering 30.

Also, in the open area 22 the conductive fiber 26 may or may not beadhered to the tank wall. In other words, in the open area 22, theconductive fiber material 26 may be partially embedded in the curedresin comprising the tank wall 21 or, alternatively, in the open area 22the conductive fiber material 26 may be completely unattached to thetank wall. The conductive fiber material 26 can be floppy andunrestrained in the open area 22.

The conductive fiber material 26 extends along the tank wall, forexample between a tank bottom 34 and a tank top 36. A tank can have asingle strip of conductive fiber material 26, or a single tank can havea plurality or many strips of conductive fiber material 26. The stripsof conductive fiber material can be spiral-wound around the tank, or canbe arranged in straight, parallel vertical strips or in any otherpattern. The present invention is not limited to any particular numberor arrangement of conductive fiber material strips.

A bolt 38 extending through the tank wall 21 can function as anelectrical feedthrough, providing an electrical connection between theconductive fiber material 26 and ground potential 40. The bolt 38 isoptional in the invention. There are many other ways to provide anelectrical ground connection to the conductive fiber material 26.

The bolt 38 can be located in an open area 22 to facilitate goodelectrical contact between the bolt 38 and the conductive fiber material26.

In operation, electrostatic charge 42 accumulates in the tank 20. Thecharge 42 can be produced by triboelectric charge separation resultingfrom movement of the liquid, as known in the art. Electrostatic chargemay also be present in the air inside the tank. The electrostatic charge42 is collected by the broken fiber tips 28 and stray fibers 29 in theopen areas 22. The charge flows through the conductive fiber material26, to the bolt 38 and then to the ground potential 40. The present tankwill collect charge from both liquid and gases in the tank. The resincovering 30 generally prevents the conductive fiber material 26 fromcollecting much charge in locations outside the open areas 22. However,in locations where the resin covering 30 is extremely thin, theconductive fiber material may collect some charge.

In some embodiments, cured resin is infiltrated into the conductivefiber material 26 in areas outside the open areas 22 (i.e. in areaswhere conductive fiber material 26 is covered by resin covering 30).

The electrostatic charge 42 may come into contact with the conductivefiber material 26 as the liquid or gases circulate inside the tank.Also, the electrostatic charge 42 will be attracted to the conductivefiber 26 and flow toward the open areas 22 due to electrostatic forces,as known in the art. When electrostatic charges are eliminated from thetank, the risk of an electrostatic-spark triggered explosion is greatlyreduced.

FIG. 4 shows a close-up view of a open area 22 according to anembodiment in which the conductive fiber material 26 is not adhered tothe tank wall 21 in the open area 22. An optional space 43 can bepresent between the conductive fiber material 26 and the tank wall 21.An impermeable film 44 is disposed between the tank wall 21 and thefiber material 26. The impermeable film 44 may be adhered to the tankwall 21 by the cured resin comprising the tank wall 21. The impermeablefilm 44 covers an area 46 that roughly matches the open area 22. Theimpermeable film 44 can be larger or smaller than the open area 22.

The impermeable film 44 can comprise a polymeric material that isresistant to and impermeable to the uncured resin used to make the tankwall 21. For example, the impermeable film can comprise polyester film,polyimide film, or polyethylene (e.g. HDPE) film. Alternatively, thefilm 44 can comprise paper, waxed paper or masking tape. The impermeablefilm 44 may also comprise a metal foil, such as aluminum, copper, brassor stainless steel foil. The impermeable film may be a piece of adhesivetape, and have a residual layer of adhesive on one or both sides. Forexample, the impermeable film 44 may have a layer of adhesive on theside facing the conductive fiber material 26.

The impermeable film 44 is an artifact of the method used forfabricating some embodiments of the present invention. As explainedbelow, the impermeable film 44 prevents uncured liquid resin frominfiltrating the conductive fiber material 26 during tank assembly. Thisis beneficial in order for the fiber material 26, broken fiber tips 28and stray fibers 29 to remain “exposed” and able to collectelectrostatic charges. Broken fiber tips 28 and stray fibers 29 willhave reduced ability to collect electrostatic charge if they are coveredby the resin covering 30.

The conductive fiber material 26 of the present invention can comprisemany different electrically conductive fibers, such as carbon fiber,intrinsically conductive polymers, conductive polymer compositematerials, or metallic fibers or wires. The conductive fiber materialcan comprise continuous fibers. The conductive fiber material comprisesa continuous electrical conductor, so that it can transport electricalcharge from the open area 22 towards the ground potential 40.

In embodiments where non-carbon fibers are used, the fibers can comprisemany different types of conductive or static-dissipative plastics orpolymers. The plastics or polymers used can be intrinsically conducting(e.g. polyaniline, polypyrrole, polyacetylene) or can be conductive dueto embedded conductive fibers, particles, carbon or nanowires (i.e.“conductive polymer composites”). Such conductive plastics and polymersare known in the art. Examples of plastics and polymers suitable for useinclude composites based on polypropylene, polyethylene, and nylon.

Conductive polymer composites can be made by incorporating many types ofconductive particles, such as carbon black, carbon nanotubes, choppedcarbon fiber, graphite powder, metal particles (e.g. aluminum powder),or metal fibers. These conductive materials can be incorporated intomany different types of plastics or polymers that can be extruded orspun into fibers suitable for use in the present invention.

The conductive fiber material 26 used in the present dissipater can havea wide range of electrical resistance values, for example in the rangeof 0.1 to 1×10⁹ ohms or 1×10³ to 1×10⁶ ohms per linear foot. Embodimentsusing carbon fiber will generally have a low resistance of under 100ohms per linear foot. Embodiments comprising conductive plastic fiberswill typically have higher resistance values, depending on the specificmaterial, and the amount of conductive material embedded in the plasticfibers. The optimal electrical resistance will depend on several factorsincluding: the desired relaxation time for removing electrostaticcharges in the tank, the rate of charge accumulation in the tank, andthe maximum tolerable amount of charge in the tank.

The fiber tips 28 and stray fibers 29 preferably have a length of atleast about 0.010″, 0.020″, 0.050″, 0.10″ or 0.25″. The distance theyproject away from the fiber material 26 will change with local electricfield strength and fluid movement forces. Typically with carbon fiber,the tips will not project further than about 0.50″ or 1″ from the yarns;however, the present invention and appended claims are not limited toany particular length of the broken fibers 28 and stray fibers 29.

Preferably the broken fiber tips 28 and stray fibers 29 are present in adensity of at least about 1, 10, 20, 100, or 200 per square inch. Thedensity of broken fiber tips and stray fibers will typically be lowerfor embodiments having large-diameter fibers (e.g. 250-1000 microns),and higher for embodiments having small-diameter fibers (e.g. 1-20microns). The density of broken and stray fibers can be increased bymechanical damage. The conductive fiber material can be abraded (e.g.rubbed with sandpaper), partially broken, partially cut or otherwisedamaged (e.g. by crushing, incising, clipping, sandblasting, laserablation, pulling, unwinding or shearing) to increase the number ofbroken fiber tips 28 and/or individual stray fibers 29. Carbon fibersare brittle and so broken fiber tips can be formed by bending the carbonfibers to a small radius of curvature.

The conductive fiber material 26 can be in the form of a fiber yarn, abraided or woven fabric of fiber yarns, or a braided sleeve of fiberyarns. The conductive fiber material can be in the shape of anelongated, flat strip. For example, the conductive fiber material can beabout 0.05″-0.10″ thick, 0.25″-6″ wide, and tens or hundreds of feetlong for example. The present invention is not limited to any particularwidth, thickness or length of the conductive fiber material.

The open areas 22 can vary widely in size. Typical sizes may be forexample in the range of 0.5″×0.5″ to 12″×12″.

The number and spacing of open areas 22 can also vary widely. A single,dozens, hundreds, or thousands of open areas 22 can be present in a tankor other structure. Also, the open areas 22 can be spaced apart byinches or feet. The present invention is not limited to any particularsize, spacing, shape, number or density of open areas 22.

FIG. 5 shows a strip of conductive fiber material 26, illustrating afirst step in a method according to the present invention. Masked areas52 a 52 b 52 c of the fiber material 26 are covered with a maskmaterial.

As noted above, the fiber material 26 can comprise carbon fiber fabricor carbon fiber braided sleeve, for example.

The masked areas can be any arbitrary shape and can be arranged in anyarbitrary pattern. The masked areas can be surrounded by unmasked areas(like masked areas 52 a, 52 c) or can extend to edges of the fiber strip(like masked areas 52 b).

The mask material can be any material that prevents infiltration intothe fiber material 26 by the resin used for constructing the tank, suchas polyester resin or epoxy. The mask material can comprise a thick gel,such as polyvinyl alcohol (e.g. 1-10% concentration) or plant-based gumsdissolved in water. The mask material can also comprise mold-releaseagents, silicone oil, natural triglyceride oils or grease for example.Preferably, the mask material is not soluble or miscible in the uncuredresin material used for constructing the tank. Also preferably, the maskmaterial is removable with a solvent or cleanser, such as water, hotsoapy water or alcohol for example.

The mask material preferably wets and infiltrates the conductive fibermaterial 26 in the masked areas 52. Also, the mask can be viscous sothat it does not flow long distances away from the masked areas 52 bycapillary action. The mask preferably stays where it is applied and doesnot run, flow or drip off the conductive fiber material 26. The maskmaterial is optionally partially or completely dried or hardened. Forexample, the mask material can be dried to a leathery, thixotropic orrubbery consistency. Preferably, the mask material is removable with thesolvent or cleanser even in a dried state.

FIGS. 6A-6F illustrate further steps for making a static dissipativetank according to the present invention.

FIG. 6A: A side view of a cylindrical mandrel 54 for making acylindrical fiber-composite tank. The mandrel 54 has an axis of rotation56. The mandrel can be made of metal and can be collapsible, as known inthe art of fiberglass tank manufacturing. A mandrel is essentially amold for making a tank. Tank mandrels are well known in the art offiberglass tank manufacturing.

FIG. 6B: Strips of masked conductive fiber material 26 are placed on themandrel. The fiber material 26 has masked areas 52, as explained above.The fiber material 26 can be adhered to the mandrel by the maskmaterial, by uncured resin used to manufacture the tank, by adhesivetape or by other means. For example, in embodiments where impermeablefilm is used as a mask, the impermeable film can comprise adhesive tapethat is used to attached the conductive fiber material to the mandrel54.

FIG. 6C: The mandrel 54 is rotated, and structural fiber strip 58 (e.g.fiberglass or carbon fiber) is wound around the mandrel 54. Thestructural fiber is soaked in uncured, liquid resin. The structuralfiber strip 58 may be unwound from a spool 59. The structural fiber 58is wound onto the mandrel until the conductive fiber strips arecompletely covered and the tank walls 21 are formed. Liquid resin mayinfiltrate into (e.g. by capillary action) the conductive fiber material26 in unmasked areas. The liquid resin is inhibited from infiltratingthe conductive fiber material 26 in masked areas 52. After winding, theresin is cured, as known in the art.

FIG. 6D: Shows a cross sectional view of the tank wall after thestructural fiber 58 has been applied. The cross sectional view is alongline 57 illustrated in FIG. 6C. The conductive fiber material 26 is seenedge-on. The tank wall 21 is formed from multiple layers of structuralfiber 58 and cured resin.

FIG. 6E: The tank is separated from the mandrel. This exposes the tankinterior and the resin covering 30. At this stage, the resin covering 30most likely covers the entire interior surface, including the maskedareas 52. However, it is possible that portions of the resin covering 30over the masked areas are pulled away by and remain adhered to themandrel 54. It is also possible that the liquid resin did not flow underthe masked areas 52 of the fiber material 26.

FIG. 6F: Resin covering sections 30 a are removed from masked areas 52of the conductive fiber material 26. The mask material facilitatesremoval of the resin covering sections 30 a because the mask materialacts as a release layer. For example, the resin covering section 30 acan have poor adhesion to the mask material, or the mask material canhave poor adhesion to the conductive fiber material 26. Removal of theresin covering sections 30 a creates open areas 22 where the conductivefiber material 26 is exposed.

The resin covering sections 30 a can be removed by hand with a scrapingtool such as a knife or wire brush. Also, high-pressure water spraycould be used. As the resin covering sections 30 a are removed, edges 32might be formed with cut, broken or sheared surfaces.

After resin covering sections 30 a are removed, the exposed conductivefiber material 26 can be rinsed and cleaned to remove remaining maskmaterial. If the mask material is made of a water-soluble material suchas polyvinyl alcohol or plant-based gums, then hot water can be used toremove the mask material. Alternatively, the tank may be put in servicewithout cleaning the mask material, and the liquids contained in thetank will wash away the mask material.

Once the conductive fiber material 26 is cleaned, the presentmanufacturing method is complete and the tank can proceed to additionalmanufacturing steps known in the art, such as attaching the tank top andbottom, or adding flanges, fittings and the like.

Also, the conductive fiber material 26 in the open areas can be abradedor otherwise damaged to increase the number of broken fiber tips 28 orstray fibers 29.

FIG. 7 shows a manufacturing step according to an embodiment in whichthe conductive fiber material 26 is wrapped in a spiral around themandrel 54. The masked areas 52 can be distributed along the length ofthe conductive fiber material 26. After the masked fiber material 26 isapplied to the mandrel, the structural fiber 58 is wrapped around themandrel as illustrated in FIG. 6C and the tank is completed in the samemanner.

The exposed conductive fibers 26 in the open areas 22 can be slightlydamaged to create broken fiber tips and stray fibers. This can beaccomplished by abrasion or sandblasting, for example.

The mask material can comprise many different substances. Preferably,the mask material has a high viscosity so that it does not flow, drip,or spread excessively. Preferably, the mask material is viscous enoughto remain the mask areas 52. Possible mask materials include polyvinylalcohol dissolved in water, ethene homopolymer, silicone oil, grease,natural oils or fats, mold release agents for composites manufacturing,natural gums (e.g. xanthan gum, guar gum, gum arabic dissolved in water)or pectins in water or other viscous or thixotropic substances. The maskmaterial may dry, thicken or harden after application, as this willfurther prevent dripping or uncontrollable flow or movement of the maskmaterial.

Preferably, the mask material is immiscible with the resin used tofabricate the tank. For example, the mask material can be immiscible inpolyester resin or epoxy.

Optionally the mask material is removable with a solvent (e.g. water)that does not damage or attack the finished, cured resin material.

In another embodiment, the impermeable film 44 is used as a mask toinhibit infiltration of the liquid resin into the conductive fibermaterial. The impermeable film 44 is impermeable to the liquid resinused to fabricate the tank. A method according to this embodiment isdescribed in reference to FIGS. 8A-8E

FIG. 8A: The conductive fiber material 26 is shown edge-on. Impermeablefilm 44 is disposed over masked areas 52 on the interior side of theconductive fiber material 26. Alternatively, the impermeable film 44 canbe disposed on the exterior side. As noted above, the impermeable film44 can comprise adhesive tape, and be made of a durable polymer such aspolyethylene or polyimide. Also, the impermeable film 44 can be used incombination with the liquid or gel mask material described above.Liquid/gel mask material can be infiltrated into the fiber material, andthen the impermeable film 44 can be adhered to the masked areas 52.

FIG. 8B: The masked fiber material 26 is disposed on the mandrel 54. Theimpermeable film 44 is disposed between the mandrel and the fibermaterial 26, but this arrangement is optional in the invention. Inanother embodiment, the fiber material 26 is disposed between theimpermeable film 44 and the mandrel 54. The fiber material 26 can beattached to the mandrel 54 with adhesive tape.

FIG. 8C: Structural fiber and liquid resin are applied over the mandrel54 and fiber material 26. The liquid resin infiltrates the conductivefiber material 26 in locations outside the masked areas 52. The resin iscured.

FIG. 8D: After the liquid resin is cured, the tank structure isseparated from the mandrel 54.

FIG. 8E: The resin covering sections 30 a are removed from over themasked areas 52. The impermeable film 44 facilitates the detachment ofthe resin covering sections 30 a. Removal of the resin covering sections30 a leaves portions of the conductive fiber material 26 exposed in theopen areas 22. The impermeable film 44 might remain attached to theresin covering sections 30 a.

In another embodiment of the present invention, impermeable film 44 isapplied to both interior and exterior sides of the conductive fibermaterial 26. FIGS. 9A-9E illustrate steps in the assembly process inwhich impermeable film 44 is applied to both sides of the fiber material26.

FIG. 9A: Impermeable film 44 is disposed over masked areas 52 on theinterior and exterior sides of the conductive fiber material 26. Liquidor gel mask material can also be applied to masked areas 52. The maskedarea 52 on the left side 47 has an impermeable film only on the exteriorside of the conductive fiber material 26.

FIG. 9B: The masked fiber material 26 is disposed on a mandrel 54. In analternative embodiment, the left side masked area 47 is attached to themandrel 54 by the impermeable film 44. A cross sectional view of suchattachment is illustrated in FIG. 13.

FIG. 9C: Structural fiber and liquid resin are applied over the mandrel54 and fiber material 26. The liquid resin infiltrates the conductivefiber material 26 in locations outside the masked areas 52. The resin iscured.

FIG. 9D: After the liquid resin is cured, the tank structure isseparated from the mandrel 54.

FIG. 9E: The resin covering sections 30 a are removed from over themasked areas 52.

The impermeable film 44 facilitates the detachment of the resin coveringsections 30 a. Removal of the resin covering sections 30 a leavesportions of the conductive fiber material 26 exposed in the open areas22. The impermeable film 44 might remain attached to the resin coveringsections 30 a. The impermeable film 44 may also remain in the finalproduct on the exterior side of the fiber material 26 (i.e. between thefiber material 26 and tank wall 21), as it may be adhered to cured resincomprising the tank wall 21, and may be difficult to remove from behindthe conductive fiber material 26.

In the present invention, the impermeable film 44 can form a tube aroundthe fiber material. FIG. 9F shows a cross sectional view of fibermaterial that is surrounded by tube of impermeable film 44. Impermeablefilm 44 can comprise two pieces, with one piece on each side of thefiber material. The impermeable film pieces 44 are wider than the fibermaterial 26, and therefore are in contact at edges 45, forming the tubethat surrounds the fiber material 26. The edges 45 can stick together byadhesive, for example in embodiments where the impermeable film 44comprises adhesive tape. Optionally, the tube of impermeable film 44 canbe filled with liquid or gel mask material.

The present invention also includes a method for attaching theconductive fiber material 26 to a surface, such as a tank interiorsurface. In this method, the mask is used to prevent infiltration of theresin that would cover the broken fiber tips 28 and stray fibers 29.

FIGS. 10A-10D illustrate the present method for attaching the fibermaterial to a tank interior surface, or other surface.

FIG. 10A: Mask material and/or impermeable film 44 is applied to maskedareas 52 of the conductive fiber material 26, as explained above. InFIG. 10A, the impermeable film 44 is shown applied to only one surface.However, impermeable film 44 can be applied to one side or two opposingsides of the fiber material 26.

FIG. 10B: The masked fiber material 26 is disposed on a surface 60 wherethe conductive fiber material is desired. The surface 60 can be theinterior surface of a tank, for example.

FIG. 10C: A resin covering 30 is applied. The covering 30 can compriseliquid resin only, or a composite of resin and structural fibermaterial. The structural fiber can be fiberglass or carbon fiber in theform of a fabric or chopped fibers, for example. The resin covering 30can be applied over the entire fiber material 26, or can be appliedapproximately only in locations outside the masked areas 52. After theresin covering 30 is applied, it is allowed to cure.

FIG. 10D: After the resin covering 30 is cured, resin covering sections30 a are removed from over the masked areas 52, thereby creating openareas 22 where the conductive fiber material 26 is exposed and capableof absorbing nearby electrostatic charge. The resin covering 30 alsobonds to the tank surface 60, thereby permanently attaching theconductive fiber material 26 to the surface 60.

FIG. 11 shows a tank with a conductive fiber material 26 adhered to aninterior surface 60 according to the method illustrated in FIGS.10A-10D. The tank will be very similar or essentially identical to thetank of FIG. 3. However, unlike the embodiment of FIG. 3, the resincovering 30 may be raised above the interior tank surface 60.

In FIG. 11, the bolt 38 is located in the open area 22 and functions asan electrical feedthrough between the conductive fiber material 26 andthe ground potential 40.

The present invention also provides a static-dissipating panel. FIG. 12shows a panel 64 according to the present invention. The panel comprisesa flat or curved sheet or strip of fiber-composite material 66. Thepanel has the open area 22 where the conductive fiber material 26 isexposed. Resin covering 30 covers the conductive fiber material 26 inlocations outside the open areas 22. The present panel 64 can be bolted,glued or otherwise attached to an interior surface of a tank or otherstructure where static dissipation is desired.

In the methods of the present invention, the conductive fiber material26 can be attached to the mandrel 54 or tank surface 60 by the mask. Forexample, FIG. 13 shows a cross sectional view of a step according to amethod in which the conductive fiber material 26 is attached to themandrel 54 or tank surface 60 with impermeable film 44 comprisingadhesive tape. The impermeable film adhesive tape 44 is wider than theconductive fiber material 26 and sticks to the mandrel 54 or tanksurface 60 in areas 68 adjacent to the fiber material 26. The spacebetween the impermeable film 44 and mandrel 54/tank surface 60 can befilled with liquid mask material. Accordingly, the impermeable film 44functions as both a mask and a means for attaching the conductive fibermaterial 26 to the mandrel 54 or tank surface 60. If the seal at edges68 is tight and does not allow entry of any liquid resin, then in someembodiments there might be no resin covering 30 a requiring removal.

Alternatively, the liquid/gel mask material can have adhesive propertiessufficient to adhere the conductive fiber material to the mandrel 54 ortank surface 60.

The present invention can provide static-dissipative objects of manyshapes for many different applications. The present invention is notlimited to making tanks or panels. The mandrel 54 can be replaced with amold of any shape for making a wide variety of composite structures orparts with exposed conductive fiber. For example, the present inventioncan be used to manufacture windmill blades, airplane components, vehiclecomponents, boat hulls or the like. The present invention can also beused in any structure where an electrically conductive surface area isdesired.

The above embodiments may be altered or combined with each other in manyways without departing from the scope of the invention. Accordingly, thescope of the invention should be determined by the following claims andtheir legal equivalents.

What is claimed is:
 1. A charge-dissipating tank, comprising: 1) a tankwall comprising structural fiber embedded in a cured resin material,wherein the tank wall has an interior surface and an exterior surface;2) a conductive fiber material embedded in the tank wall; 3) an openarea disposed on an interior surface of the tank and at least partiallyoverlapping the conductive fiber material, wherein the conductive fibermaterial is exposed in the open area, and wherein the conductive fibermaterial is covered with resin in locations outside the open areas. 2.The tank of claim 1 wherein the exposed conductive fiber material has abroken fiber tip and stray fiber density of at least 1 per square inch.3. The tank of claim 1 wherein the exposed conductive fiber material hasa broken fiber tip and stray fiber density of at least 20 per squareinch.
 4. The tank of claim 1 further comprising an impermeable filmlayer disposed between the conductive fiber material and the tank wall,and at least partially overlapping with the open area.
 5. The tank ofclaim 4 wherein the impermeable film material is adherent to the tankwall.
 6. The tank of claim 1 wherein the resin is infiltrated into theconductive fiber material in locations outside the masked areas.
 7. Amethod for making a charge-dissipating object, comprising the stepsof: 1) applying a mask material to a conductive fiber material, whereinthe mask material inhibits infiltration of the conductive fiber materialby resin, and wherein some areas of the conductive fiber material do nothave mask material applied 2) disposing the conductive fiber materialand mask on a mold; 3) disposing structural fiber and liquid resin onthe conductive fiber material and the mold, such that the resininfiltrates unmasked areas of the conductive fiber material; 4) curingthe resin; 5) releasing the conductive fiber material, structural fiberand cured resin from the mold; 6) removing the mask material or resincovering from the conductive fiber, such that the conductive fiberbecomes exposed in the previously masked areas.
 8. The method of claim 7wherein step 1 includes applying an impermeable film to one side or twoopposing sides of the conductive fiber material.
 9. The method of claim8 wherein the impermeable film mask comprises a tube around theconductive fiber material.
 10. The method of claim 8 wherein the objectcomprises a tank, and the mold comprises a tank mandrel, and wherein, instep 6, the mask material or resin covering is removed from an insidesurface of the tank.
 11. The method of claim 7 wherein the mask materialcomprises a liquid or gel infiltrated into the conductive fibermaterial, and an impermeable film in contact with the liquid or gel. 12.The method of claim 7 wherein step 1 includes applying a liquid or gelmask material to the conductive fiber material such that the liquid orgel infiltrates into the conductive fiber material.
 13. The method ofclaim 7 wherein step 2 is performed before step
 1. 14. The method ofclaim 7 wherein step 6 includes removing a resin covering from thepreviously masked areas.
 15. The method of claim 7 wherein step 3includes wrapping the structural fiber material around the mandrel andon top of the conductive fiber material.
 16. The method of claim 7wherein the mask material is water-dispersable, and step 6 includesrinsing the open areas with water.
 17. A method for applying aconductive fiber material to a surface, comprising the steps of: 1)applying a mask to a conductive fiber material, wherein the mask rendersthe conductive fiber material uninfiltrateable by resin, and whereinsome areas of the conductive fiber material do not have mask applied; 2)disposing the conductive fiber material and mask on the surface; 3)applying liquid resin on the masked conductive fiber material, such thatthe resin infiltrates unmasked areas of the carbon fiber material andadheres the fiber material to the surface; 4) curing the resin; 5)removing the cured resin and mask from masked areas of the conductivefiber, such that the conductive fiber becomes exposed in the previouslymasked areas.
 18. The method of claim 17 wherein step 1 includesapplying an impermeable film to one side or two opposing sides of theconductive fiber material.
 19. The method of claim 18 wherein theimpermeable film mask comprises a tube around the conductive fibermaterial.
 20. The method of claim 18 wherein the impermeable filmcomprises adhesive tape, and the adhesive tape mask functions to attachthe conductive fiber material to the tank surface.
 21. The method ofclaim 17 wherein the mask material comprises a liquid or gel infiltratedinto the conductive fiber material, and an impermeable film in contactwith the liquid or gel.
 22. The method of claim 17 wherein step 1includes applying a liquid or gel mask material to the conductive fibermaterial such that the liquid or gel infiltrates into the conductivefiber material.
 23. The method of claim 17 wherein step 2 is performedbefore step
 1. 24. The method of claim 17 wherein step 6 includesremoving a resin covering from the previously masked areas.
 25. Themethod of claim 17 wherein the mask material is water-dispersable, andstep 6 includes rinsing the open areas with water.
 26. The method ofclaim 17 wherein the surface is an interior surface of a tank.
 27. Acharge-dissipating tank, comprising: 1) a tank sidewall comprisingstructural fiber embedded in a cured resin material; 2) a conductivefiber material embedded in the cured resin; 3) a open area disposed onan interior surface of the tank sidewall and at least partiallyoverlapping the conductive fiber material, wherein the conductive fibermaterial is exposed in the open area, and wherein the conductive fibermaterial is covered with cured resin in locations outside of the openareas, and wherein the exposed conductive fiber material has a brokenfiber tip or stray fiber density of at least 10 per square inch.
 28. Thetank of claim 27 further comprising an impermeable film layer disposedbetween the conductive fiber material and the tank wall, and at leastpartially overlapping with the open area.